Passive infrared detection systems are known for the detection of intruder presence in a protected area, and in general include a detector, and an optical assembly for focusing infrared radiation onto the detector, and electronic signal processing circuitry for providing an output indication of intruder detection when predetermined threshold conditions have been met.
The range of the infrared detection system is determined by the size of the detector element and the focal length of the optics, the maximum range being defined as that beyond which the optical image of an intruder or other detectable object does not fill the area of the detector element. Up to the maximum range, the radiation received by the detector is constant because the change in the received radiation is offset by the change in image area thereby resulting in substantially constant detector sensitivity with range, up to the maximum range of the system. Beyond the defined maximum range, the sensitivity will decrease in conformance with the inverse square law which provides that the intensity of radiation decreases in proportion to the square of the distance from its source.
Detection systems of known construction are usually designed for an intended range. For installations where there are different range requirements, systems designed for the respective ranges must be separately constructed and provided.